Effects of Atrazine and Dichlorvos on Soil Microbial Biomass and Snail Toxicity: A Microcosm Study

Abstract

Agricultural pesticides like Atrazine and Dichlorvos persist in soil; disrupt microbial communities and soil-dwelling organisms such as snails, affecting key ecosystem processes. These chemicals reduce microbial biomass indicators (MB-C, MB-N, MB-P) and biodiversity, highlighting the need for integrated studies on their combined ecological impacts in soil systems. This study investigated the effects of Atrazine and Dichlorvos on microbial biomass carbon (MB-C), nitrogen (MB-N), phosphorus (MB-P), soil microbial populations, and snail toxicity in a controlled microcosm. Surface soils (0–15 cm) from pesticide-free plots at the Federal University of Petroleum Resources, Effurun, Nigeria, were characterized and treated with manufacturer-recommended doses of both pesticides for 8 weeks. Contaminated and uncontaminated soils were assessed weekly for MB-C, MB-N, MB-P, and microbial population changes using aerobic plate counts. Also, the acute effect of both pesticides on snails was assessed using a 14 day soil bioassay following standard protocol. The pristine soil was moderately acidic (pH 6.70), with a total organic carbon, total nitrogen, phosphate and nitrate contents of 3.316%, 0.3029%, 26.32 mg/kg and 36.28 mg/kg, respectively. Baseline microbial counts were 6.30 × 10⁷ CFU/g for bacteria and 1.41 × 10⁵ CFU/g for fungi. Both pesticides caused initial reductions in MB-C, MB-N, and MB-P, followed by recovery by day 28, when MB-C (293 µg/g), MB-N (18.5 µg/g), and MB-P (11.4 µg/g) exceeded control values. Phosphate-solubilizing bacteria and actinomycetes declined notably. Acute toxicity tests using Helix aspersa showed Dichlorvos to be far more toxic (LC₅₀: 1.13 mg/kg) than Atrazine (LC₅₀: 10.04 mg/kg). This study demonstrated distinct pesticide impacts on soil microbial functioning and non-target invertebrates.